Answer
$$\frac{d^2y}{dx^2}=6\Big(\cos(3x^2)-6x^2\sin(3x^2)\Big)$$
Work Step by Step
$$\frac{dy}{dx}=(\sin(3x^2))'=\cos(3x^2)(3x^2)'=\cos (3x^2)\cdot 6x=6x\cos(3x^2)$$
$$\frac{dy^2}{dx^2}=\frac{d}{dx}\Big(\frac{dy}{dx}\Big)=(6x\cos(3x^2))'=
(6x)'\cos(3x^2)+6x(\cos(3x^2))'=
6\cos(3x^2)+6x(-\sin(3x^2))(3x^2)'=
6(\cos(3x^2)-6x^2\sin(3x^2))$$
